(a) FIELD OF THE INVENTION
The present invention relates to endoscopes and more particularly to an optical system for endoscopes.
(b) DESCRIPTION OF THE PRIOR ART
Generally, an endoscope incorporates an observing optical system comprising an objective lens and an image guide and an illuminating optical system comprising an illuminating lens and a light guide in the body thereof. In such an endoscope, variation in the distance between the distal end portion of the body and the surface of an object brings about variations in the distance from the exit end face of the light guide to the surface of the object and the distance from the surface of the object to the entrance end face of the image guide. When the distances are varied as mentioned above, brightness in the field of view will be rapidly changed. Usually, therefore, when the object is observed from a short distance, the field of view is liable to be extremely illuminated and conversely when the object is viewed from a distance in some extent, the field of view tends to be dark in excess.
Thus, in order to solve this problem, consideration has been given to provide a stop in the above optical system. However, it has been impossible to incorporate a conventional stop mechanically constituted, which is large in size, in the body of an endoscope.